I'm a big fan of modularity. For some discussion of modularity concepts you might enjoy reading "FXBus - an Effects Experimenter's Delight!" from over 20 years ago in 2000 at geofex.com. See:
http://geofex.com/Article_Folders/FXbus/fxbus.htm
It's about effects, not tube amps, but it does discuss modularity as a concept. Modularity involves some underlying concepts you'll need.
You need defined interfaces. If you want to swap things in and out, all the things you swap in must have corresponding input and/or output and power features and requirements so swapping is even possible without rebuilding. The list of seven concepts for the FxBus is an example of this idea, mildly customized for effects circuits.
As an example, take guitar input preamp circuits. They all need a high input impedance, generally 1M or more. They need a significant gain, often 30db or more; they need to be able to drive any of your tone/volume modules' input impedance. That last may mean that all of your tone/volume modules have an impedance level requirement, or that all of your preamp modules need buffers to drive any and all of the tone/volume modules. They all need to work from the power supply and grounding scheme in the amp they will be swapped into/out of. They all need to play nicely with the other modules in the modular amp, including not broadcasting noise or polluting power and ground into the rest of the amp.
By now you may be picking up the idea that to get to just modular preamps, you have to know the interface details intimately, and be able to adapt a new/different preamp concept to get it to share the common interface. And to define the interfaces, you must know enough about the possible circuits to set interface requirements so they're possible and practical to meet. Some modules will need additional controls or switches, etc., so you typically have to make provisions for these in a way that does not get in the way of other modules' working. Getting to modularity requires you to know more about many possible circuits, not less, just so the swappable modules will work together.
What you're proposing amounts to the FxBus for guitar amps. I am fond of the idea; but I think it's going to be a tough thing to design.
At a moderately deeper level, guitar amp circuit design is already modular on the circuit level. A large number of the preamp circuits, tone/volume circuits, reverb circuits, tremolo circuits, channel switching circuits, phase inverter circuits and power amp circuits have already been designed. Guitar amp designers usually have a more or less encyclopedic knowledge of amplifier designs from the past and mix/match these circuits at a thought/schematic level, modifying circuit values a bit based on experience and luck.
There is a danger lurking under the idea of modularity and interfaces. Guitarists are famously picky about things they either hear or think they hear. The notion of a "guitar amp" is so ingrained into the music world that you find flame wars over the age of capacitors and the brand of resistors and tubes. If changing one resistor for another of the same value but a different brand (let alone swapping the resistor end-for-end, which some people think is audible) then any change you make to the interfaces of a circuit to get modules to be swappable is likely to change (and some would say damage) the sound. A modular setup may work fine for your ears, but it's the kind of thing that in the music world you do behind closed doors and wash your hands afterwards.
With that out of the way, here are a few thoughts on your questions.
gerp124 wrote: ↑Sat Oct 05, 2024 5:25 pm
My design philosophy is that I want to break components down into small functional units that I can mix and match- that way I don’t have to make so many decisions at once- I can choose a pre-amp design to start with, then a power amp design, then a cabinet design, and from there, I can build new ones and combine them to see what I like.
Good idea! Dang, it's going to be a lot of work to get something usable, though.
I think the big challenge is that I seem to be looking for a Goldilocks resource- everything I'm finding is either cookbook procedural, or overly theoretical. I think I really need something specific enough to be close to buildable- referencing specific schematics, but also theoretical enough to be able to guide some design modification so that I can adapt the pieces to interface the way I want.
I don't think that exists, at least not in that form. I'm an EE by training and profession, and I kind of like boring all the way down to the bare metal; I made a living designing power circuits, logic circuits and software. What I did for personal enjoyment was learning and designing musical electronics. I had to be largely self taught at effects and guitar amps, so I read everything I could get my hands on about them, for some decades. It is likely that Blencowe and Aikens and a few others are writing as close to what you are wanting as you're going to get. I started with the Radiotron Designer's Guide and the MIT Radiation Labs amplifier series and worked my way up and through the plethora of pre-existing amp designs until I could recognize at a glance what a tube circuit was doing. This took a long time.
I was always fond of the refrain from Kenny Rogers' "The Gambler"; you gotta know when to hold 'em, know when to fold 'em. It's a long road.
1) Is there a good resource that discusses pre-amp topologies?
Maybe. Lots of people like Blencowe and Aiken. I've never read them, only in excerpts others post on line. The essence of canonical guitar amp preamps is the common cathode triode circuit, sometimes buffered by a cathode follower. This is tinkered infinitely. A very, very few amps will use a pentode input tube for high gain. These are the outliers. Many amp designers don't bother with pentode inputs at all.
Guitar amp preamps will need to take "guitar level" signals [yep you have to know what that is for typical guitars] and amplify them up until they can drive the power amplifier section input at least enough to drive it to full non-clipping power. Many guitarists talk long and loud about power amp distortion, so you may have to overdrive the power amp as well. Your preamp will also need some kind of tone and volume controls, and may need its own gain control and possibly need to drive a master volume of some kind – that’s another module or modification to the power amp itself. The commonest tone/volume controls insert about 20db of signal loss, so you will probably need some kind of recovery stage to get back the losses in the tone/volume controls. Finally you’ll need the preamp to be able to drive any effects circuits you attach, any effects loop for outside the amp itself, and any internal channel/gain switching.
2) What are the strategies for channel switching?
1. Make two (or more; don’t see this one very much) different preamp circuits and switch between the two entire circuits.
2. Make one fancier, more complicated preamp circuit and allow switching in more gain and/or different control settings with the channel switch. There is a lot of tinkering in the guts of the circuit to make this one work.
3) How should an effects loop work?
It should provide “normaled” send/receive jacks so you can insert the send/receive lines to the effect(s). It puts requirements on either the amp to buffer the outgoing/send signal so that any unspecified effect does not load down the guitar amp’s signal path, or on which effects you can use without loading down the path.
A lot of this is wrapped up in the idea of a guitar-level vs a line-level effects loop. It may or may not need a level control on the “send”. Depends on what the input level expectations of the perhaps-unknown effects are. These often devolve into whether the effects you attach expect guitar level (100mV-ish) input signals or line level (770mV to 2V) signals on their inputs. For extra credit, make this switchable.
It may need a return level control, depending on what the possibly-unknown effect output level is. Switchable is nice. A guitar-level effects loop return will generally need what amounts to a simple preamp; you attenuate the nicely preamplified guitar signal down to guitar level again, send it through an effect, and get it back at guitar level. Now you have to amplify it back up to drive the rest of the amp after the effects loop.
4) How does a spring reverb loop differ from an effects loop?
A spring reverb is a small power amplifier that has to drive the reverb tank input coil like a speaker. This can take currents of up to about 100ma for some reverb tanks. Some tanks have ~8 to 16-ish ohm coils. Then what you get back is again down at guitar-ish level again, so the reverb return must be (pre)amplified back up to the level where it can go back into the amp and be heard/mixed properly.
5) How does all of this interface with the power amp?
At the simplest level, the power amps used in guitar amps expect a voltage signal able to drive a tube gain stage and a phase inverter. The inputs are tube grids, and high impedance. Nearly all guitar amps mix all the preamps and the reverb together into one signal to drive the power amp. Simple resistor mixers are almost universal. You wind up with one signal voltage, all prepared and ribbon-wrapped to drive the power amp. The exceptions to this are usually in the form of tremolo. Some tremolos change the signal back in the preamp, and some modify the gain of the power amp itself.
And, to be overly obvious, it depends on the power amp. Guitar power amps are not nearly as standardized as, for instance, hifi power amps. The hifi industry faced many of these issues and came up with “line level”, which is about 770-1000mV rms at a 600 ohm impedance level. This was one of those interface definitions I mentioned above. They said as a practical matter that their preamps would amplify up to at least this level, and their power amps could be driven to full power by this level.
The guitar amp world never got there. There isn’t an agreed-to signal definition for driving guitar power amps. There are de-facto standards, largely that the phase inverter must supply enough grid signal to the output tube(s) to fully drive it. Given that there are only a few different kinds of usable output tubes and then single/push-pull outputs, the signal levels tend to converge. This is a place where encyclopedic knowledge of previous guitar amps is a huge help.
For what you’re describing, I suggest that you adopt the concept of line-level and make your preamps put it out, and your power amps take it in. This will be far less work than designing your own power amp interface level.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
